The material presented as background information in this section of the specification is not necessarily prior art.
Assemblies of lithium-ion battery cells are finding increasing applications in providing motive power in automotive vehicles and in many other commercial applications requiring low weight, highly-efficient electrical power sources. Lithium-sulfur cells and other lithium-electrode containing cells, utilized with anhydrous electrolytes, are also candidates for such applications.
Each lithium-ion cell of the battery is capable of providing an electrical potential of about three to four volts and a direct electrical current based on the composition and mass of the electrode materials in the cell. The cell is capable of being discharged and re-charged over many cycles. A battery is assembled for an application by combining a suitable number of individual cells in a combination of electrical parallel and series connections to satisfy voltage and current requirements for a specified electric motor or other application. In a lithium-ion battery application for an electrically powered vehicle, the assembled battery may, for example, comprise up to three hundred individually packaged cells that are electrically interconnected to provide forty to four hundred volts and sufficient electrical power to an electrical traction motor to drive a vehicle. The direct current produced by the battery may be converted into an alternating current for more efficient motor operation.
The batteries may be used as the sole motive power source for electric motor-driven electric vehicles or as a contributing power source in various types of hybrid vehicles, powered by a combination of an electric motor(s) and a hydrocarbon-fueled engine.
In these automotive applications, each lithium-ion cell typically comprises a negative electrode layer (anode, during cell discharge), a positive electrode layer (cathode, during cell discharge), a thin porous separator layer interposed in face-to-face contact between parallel facing electrode layers, and a lithium-containing, anhydrous liquid, electrolyte solution filling the pores of the separator and contacting the facing surfaces of the electrode layers for transport of lithium ions during repeated cell discharging and re-charging cycles. Each electrode is prepared to contain a layer of an electrode material, typically deposited on a thin layer of a metallic current collector.
For example, the negative electrode material has been formed by depositing a thin layer of graphite or lithium titanate particles, often mixed with conductive carbon black, and a suitable polymeric binder onto one or both sides of a thin foil of copper which serves as the current collector for the negative electrode. The positive electrode also comprises a thin layer of resin-bonded, porous particulate, lithium-metal-oxide composition bonded to a thin foil of aluminum which serves as the current collector for the positive electrode. Thus, the respective electrodes have been made by dispersing mixtures of the respective binders and active particulate materials in a suitable liquid, depositing the liquid-solid mixture as a layer of controlled thickness on the surface of a current collector foil, and drying, pressing, and fixing the resin-bonded electrode particles to their respective current collector surfaces. The positive and negative electrodes may be formed on current collector sheets of a suitable area and shape, and cut (if necessary) and folded or otherwise shaped for assembly into lithium-ion cell containers with suitable porous separators and a liquid electrolyte.
LTO desirably has certain advantages, like high cut voltage (e.g., cut-off potentials relative to a lithium metal reference potential) that desirably minimizes or avoids undesirable coatings of solid electrolyte formation. Furthermore LTO is a zero-strain material having minimal volumetric change during lithium insertion and de-insertion, thus enabling long term cycling stability, high current efficiency, and high rate capabilities. Such long term cycling stability, high current efficiency, and high rate capabilities are particularly advantageous for power battery and start-stop battery use.
However, while LTO is a promising anode material for high power lithium-ion batteries, providing extremely long life and exceptional tolerance to overcharge and thermal abuse, in certain circumstances, when used with certain cathode materials and electrolytes, LTO may potentially have certain disadvantages. For example, it has been observed that the Li4Ti5O12 particles can interact with incidental water molecules in an operating lithium cell and generate gas within a battery cell. The gas may comprise any of hydrogen, carbon monoxide, carbon dioxide, and gaseous hydrocarbons. It would be desirable to improve LTO anode materials to suppress gas formation, while employing the desirable aspects of the LTO material that provide durable batteries with sustained high capacity, high discharge rates, and long life.